Facies Changes and Hydrocarbon Presence in Offshore Indus Basin, Pakistan
Pakistan Offshore Exclusive Economic Zone extends over an area of almost 265,650 sq. km, bordering Pakistan coastline of about 825 km length between Iran border in the west and Indian border In the east. Pakistan Offshore is divided into two major divisions: (i) Offshore Indus basin, and (ii) Offshore Markran basin; having Murray Ridge/Owen Fracture as the dividing line between the two. Offshore Indus Basin lies between Murray Ridge and Indian border and Is divided tectonically Into three units: (1) An Offshore Depression on the west between Murray Ridge and the Hinge-Line, (2) The Offshore Karachi Trough Platform between the Hinge-Line and Karachi Trough shoreline, and (3) The Offshore Thar Slope Platform or Indus River Deltaic area.
Nine wells were drilled in offshore Indus basin, three wells near Karachi Trough shoreline (Dabbo Creek, Patlanl Creek and Korangl Creek), one well on the Platform (Karachi South A•1) and five wells In the Depression (Indus Marine A-1, B-1 and C-1; PakCan-1 and Sadaf-1). Oil and gas shows and traces were recorded In all wells but gas was only discovered in DST-3 Miocene sandstone horizon of PakCan-1 In January 1986 with gas flow rate 3.7 MMCF/day. The presence of gas has opened up vast avenue of further exploration and drilling and confirms the presence of hydrocarbons in offshore Indus basin. Cretaceous sediments become dominantly shaly/marly towards the west from shoreline in offshore Indus basin and are deeply buried. They can be explored and drilled in onshore at comparatively shallow depths. So Paleocene, Eocene, Oligocene and Miocene limestones/sandstones seem to be the main hydrocarbon objectives In offshore Indus basin. Isopachs and facies maps of Eocene Kirthar Formation, Oligocene Narl Formation and Miocene Gaj Formation indicate that Indus river came Into existence after Eocene bordering the Sulaiman and Kirthar mountain ranges in the east and built a delta advancing southward resulting in the present position. The maximum thickness of sediments In offshore Indus basin is calculated from seismic sections as greater than 11,000 metres and reflects sedimentation since rifting of the Indian margin In Cretaceous and earlier times. However, the fan sedimentary sequences have been deposited since Oligocene as a consequence of Himalayan uplifts and sea level changes